This invention relates to a process for preparing concentrated stable liquid dye solutions of CI Direct Yellow 11 (Paper Yellow 11). Paper Yellow 11 is an intense yellow stilbeneazo(xy) dye that is very substantive to cellulose. The structure assigned to this dyestuff is ##STR1##
It is produced by a well known and well publicized process. The industrial process was described in BIOS Report 1548 after world War II. It is based on the self-condensation of 5-nitro-ortho-toluene sulfonic acid in the presence of a concentrated alkali metal hydroxide solution. The products obtained are the alkali metal complexes of the stilbeneazo(xy) dye.
The 5-nitro-ortho-toluene sulfonic acid is neutralized by the base, usually sodium hydroxide and the solution is heated to 50.degree.-90.degree. C. for several hours to promote the condensation. The excess base is neutralized with sulfuric or hydrochloric acid. The dye is filtered from the reaction mass. It is marketed as the wet cake or the cake is dried.
The neutralized dye is a sodium salt complex of the chromophore and is only sparingly soluble in water (1-5%). With other stilbene dyes the poorly soluble salts are converted to more soluble salt to achieve improved solubility. This would be accomplished by precipitating the chromophore in its free acid form by the addition of acid. This free acid form is then converted to a more soluble salt by dissolving it in an appropriate base. However, this standard method for improving solubility to form dye solutions is inappropriate for Paper Yellow 11 (Direct Yellow 11) as its free acid form has extremely poor filtration characteristics. The salts formed during the acid neutralization with sulfuric or hydrochloric acid (sodium sulfate or sodium chloride) form a viscous slime. This slime could not be filtered using conventional equipment. Because of this filtration problem, the salts could not be removed from the resultant wet cake. Removal of the salt is necessary for the formation of liquid Paper Yellow 11 concentrates.
Concentrates of Paper 11 are desireable for pulp dyeing in paper manufacture, but such concentrates have not been readily available.
There have been several methods proposed heretofore for obtaining the desired Paper Yellow 11 concentrates. They have primarily been based on redesign of the condensation reaction to directly provide the dye as a concentrated aqueous solution. These methods have had several shortcomings due either to instability of the final product during industrial storage and economical or ecological drawbacks.
U.S. Pat. No. 3,905,949, issued in 1975 to E. I. duPont describes a method of condensation in which the alkali metal base is lithium hydroxide. The final product is a highly concentrated, water-soluble dye paste. This soluble lithium salt complex is more substantive than the sodium salt complex. The disadvantage of this lithium-based paste product is that it does not form sufficiently stable dye solutions. On storage, the concentrated solutions within a short time, at elevated temperatures, formed gels and tended to crystallize or flocculate at slightly below room temperature (5.degree.-15.degree. C.). Such problems with "pot life" seriously interfere with industrial use of this invention.
It has also been proposed, in German Offenlegungsschrift No. 2,820,487, that the base-catalyzed 4-nitrotoluene-2-sulfonic acid be carried out in the presence of the reaction products of ammonia and ethylene oxide, sodium or lithium hydroxide. In German Offenlegungsschrift No. 3,046,450 this condensation takes place in the presence of a primary, secondary or tertiary amine and lithium hydroxide. It has been noted that at least some of the sulfo groups on the chromophore prepared in this manner are in the form of alkali metal salts.
German Offenlegungsschrift No. 3,110,261 describes the condensation in the presence of sodium hydroxide and alkanolamines in an aqueous/alcoholic medium. The organic solvent system including glycol-ethers as well as conventional alcohols provide stable dye solutions but has been ecologically faulted for the relatively large amounts of the glycol-ethers discharged in the waste waters after paper dyeing.
Another drawback has been the reduced solubility and poor stability of concentrated dye solutions because of the salt content of the finished dye solutions. Since the condensations are carried out in strongly basic media, the neutralization of these bases produces enough salts to adversely affect the stability and solubility of the dye. Desalting steps have been proposed.
In German Offenlegungsschrift No. 2,451,219, sodium ions are precipitated (after acidification) by means of the relatively expansive hexafluorosilicic and replaced by cations which improve solubility. This method requires an additional filtration step to remove the precipitated sodium hexafluorosilicate.
European Patent Application No. 53,220 offers a method for preparing low-electrolyte dyes, by first converting the dye to lipophilic amine salt which is separated from the highly electrolytic salt-containing reaction mass. The separated lipophilic amine salt is then reacted with diethanolamine to form the diethanolammonium salt. The lipophilic amine is recovered and recycled. This method is expensive and technically complicated in that two resalting stages are included, each requiring a difficult phase separation.
It is an object of this invention to provide a process for converting the product of the standard sodium hydroxide catalyzed condensation of 5-nitro-ortho-toluene sulfonic acid to liquid, water-soluble concentrates of Paper Yellow 11 (C.I. Direct Yellow 11) that are stable for storage as concentrates and in dyeing solution dilution.